This invention relates to an identification system for wireless electronic identification and data transmission, comprising at least one electronic label carrying data, which data can be read electromagnetically and wirelessly by an electronic reading unit. The electronic label, sometimes referred to as a responder, includes one or more active circuits, the feed energy for which is supplied by an interrogation field generated by the reading unit. Accordingly, the labels do not need an internal power source.
A system of this kind is disclosed in applicants' Netherlands patent 176404. The known system can operate according to the absorption principle. The absorption principle is based on the fact that a resonant LC circuit can absorb energy from an external field. The external field can be formed in two ways. First the external field may be a substantially magnetic A.C. field with the responder using a coil in an LC circuit to absorb energy from an external magnetic field. This may be, for example, a coil wound around a ferrite rod or an air coil. Second the external field may be a substantially electrical A.C. field with a capacitor plate in the responder having such a form as to effect a capacitive coupling with the electrical A.C. field of the reading unit, so that energy be absorbed from the external field The latter principle is disclosed in applicants' Netherlands patent application 8702426.
The absorption principle will now be described in more detail, assuming by way of example that the reading unit, sometimes referred to as an interrogator or transmitter/receiver, generates a substantially magnetic A.C. field. The magnetic A.C. field generated by the reading unit, referred to hereinafter as the primary field, generates a voltage across the LC circuit of a responder present in the field, which voltage is generated in the LC circuit of the responder owing to the change in flux of the primary field. Also, the voltage across the LC circuit is proportional to the resonant rise factor, sometimes referred to as the quality factor Q, of the LC circuit.
The electrical energy present in the LC circuit is alternately stored in the coil (1/2 LI.sup.2) and in the capacitor (1/2 CV.sup.2), and during the oscillation of the LC circuit the energy stored goes back and forth between the coil and the capacitor. This energy comes from the primary field and has been absorbed by the coil of the LC circuit of the responder from the primary field in a number of successive periods. The energy content of the primary field is thus decreased. This is expressed in an increase of the losses (or damping) of the antenna coil of the reading unit. The increase in damping in the primary circuit can be well detected electronically, as described in the above patent and in the patent application. Upon closer consideration, there is an energy balance. The absorption of energy from the primary field causes the energy content of the LC circuit of a responder to keep increasing. On the other hand, the energy content of the LC circuit decreases as a result of losses in the LC circuit and losses resulting from the load formed by (electronic) circuits of a responder connected to the LC circuit. In the condition of equilibrium, the energy output due to losses equals the energy input absorbed from the primary field by the LC circuit. In non-stationary situations, such as when the primary field is switched on or off, the LC circuit of a responder behaves as an energy buffer. When the primary field is switched on in one step, this will cause an oscillation in the LC circuit of a responder present in the field, with a gradually increasing amplitude. This results in the absorption of a net amount of energy until the situation of equilibrium is reached. Conversely, the LC circuit will continue to oscillate when the primary field is switched off in one step. The decrease in oscillation is then determined, and so is the increase after switching on, by the magnitude of the losses, i.e. by the quality factor Q. After Q periods, the amplitude of the oscillation has decreased to a factor l/e of the initial value.
Accordingly, when the Q factor is sufficiently high, the LC circuit in the responder continues to oscillate for a considerable period of time after the primary field has been switched off, whereby the coil generates its own secondary magnetic field. By means of a receiver, either separate or connected to the antenna coil of the reading unit, the secondary field present after the transmission signal of the reading unit has been switched off can be detected.
One advantage of detection of the secondary field is that the transmission coil generating the primary field is then switched off. In the known identification systems, the primary field is always present, and the responder signal received must be filtered. However, the signal can only be filtered to a certain extent. There will always continue to be some noise because of the primary field being generated and being continuously present.
In European patent application 0157095, a shop lifting detection system is described, which is based on the detection of anti-theft labels during time intervals in which the primary field is temporarily switched off. However, these known anti-shop-lifting systems only permit the detection of the presence of a responder. It is not possible to transmit data to the reading unit and/or to identify individual responders.
It is an object of the present invention to provide a system combining the advantages of the shop-lifting detection systems described above with the advantages of the known identification systems. It is a more general object of the present invention to provide a detection system which operates effectively and reliably and, in addition to permitting the detection of the presence of a label, enables its identification and, if desired, data transmission between a label and the reading unit.
These and other objects are achieved, in accordance with the present invention, by providing an identification system for wireless electronic identification and data transmission, comprising a reading unit and at least one electronic label comprising a resonant circuit and carrying data stored in a memory device of the label and capable of being read wirelessly when the label is introduced into a high-frequency interrogation field formed by a transmitter of the reading unit, characterized in that the reading unit includes a control device which, in operation, periodically switches the interrogation field on and off and in the time intervals when the field is switched off switches on a receiver capable of detecting a signal transmitted by a label, said at least one label including a modulator which, in the time intervals when the field is switched off, can modulate the quality factor Q of the resonant circuit in dependence upon the data stored in the memory device.